An FM/CW radar system often uses a linear frequency modulation to provide accurate range information. The resolution of the ranging information is directly dependent on the linearity of the transmit signal. However, the performance output of a VCO used to generate the radar signal is typically non-linear. Therefore, special effort must be taken to insure that the central voltage for the VCO produces the desired linear modulation ramp output slope.
A prior approach to producing a linear frequency modulation signal from a VCO uses an open loop digitally driven modulator (DDM) as shown typically in FIG. 1. The open loop approach operates by adjusting the slope of the output voltage every clock cycle to match the desired output curve. Typically, at each clock cycle a counter 103 counts clock signals from a clock 101 to select previously stored data values from a programmable read only memory (PROM) 105. These values are converted to an analog signal by a digital to analog convertor (DAC) 107. The analog signal is then typically sent through a buffer or inverter 109 and 111 to an integrator 113, where a maximum voltage corresponding to the top point in the frequency signal detected at 115, and a minimum voltage corresponding to the bottom point in the frequency signal is detected at 117. At the maximum and minimum voltages on the curve, the comparators 115 and 117 toggle the flip-flop 119. The output of the flip-flop 119, via control line 121, inverts the voltage to the integrator 113 and allows a triangle modulation. The one shot reset 123 synchronizes the counter to the output curve.
An open loop DDM has the following advantages over a standard breakpoint generator:
1) An arbitrary modulation wave form may be produced with the DDM while a breakpoint generator requires the slope of the curve to be a monotonic function. PA1 2) For high linearity of non-linear tuning curves, the hardware complexity of the DDM is much than the breakpoint generator. PA1 3) For volume production, the DDM is more amendable to automatic test equipment for deriving the desired tuning curve output rather than the breakpoint generator.
While the use of an open loop DDM solves a number of problems, there are still serious disadvantages with this approach. Each production unit must be individually tested and calibrated to determine its output curve and set the desired output curve function accordingly. In addition, the output curve of each production unit changes in response to operating conditions, most notably temperature, and may also change over time as the unit ages.
Therefore, there is a need for a VCO control apparatus that does not require the individual testing and calibration of each production unit. There is an additional need to provide a consistent output over a range of changing operating conditions and temperatures. There is a further need to eliminate or automatically compensate for inaccuracies which may arise in various components within the apparatus.